6809
shorter time; products, 2g and 2h) after 42% conversion no racemization of l g was observed, Similarly with the optically active l k (Pyrex) no racemization was observed after 24z conversion.17 Clearly the 6 process, occurring at short wavelength only, does not involve a comparatively long-lived biradical capable of reversion to starting material, as has been observed in the analogous type 11 reaction^.^ On the other hand the y process, occurring at long wavelength, may well be of’this type. The evidence described requires that a higher excited state of an aromatic thione be long-lived enough for chemical reactivity to compete with photophysical processes. That such is possible has already been demonstrated for thiobenzophenone (S2; 7 lo-” sec) an upper state of which exists long enough for intermolecular reaction; l8 such phenomena may be a consequence of the large S,-Ss separation. Efficient intersystem crossing (Sl-+Tl), as indicated by the observation of phosphorescence excludes the possibility of S1 and TI being the two reactive states.
a
TWIST ANGLE 42’
H
b
Figure 1. Calculated geometry for transition state for (a) conrotatory and (b) disrotatory interconversion of cyclobutene and 1,3butadiene.
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The nonplanarity of the transition state was subsequently confirmed by an ab initio calculation by Buenker, et al. l o While we7 and Buenker, et al.,1° also arrived at estimates for the differences between the activation energies for the two pathways, both of these were open to criticism in that assumptions were made concerning the geometry of the disrotatory transition state. We have now reinvestigated the problem using a recently im(17) The expected cyclopentanethiol from this irradiation has been proved version of M I N D 0 ( M l N D 0 / 3 11) together with characterized. The stereospecificity of its formation is being studied. a much superior geometry program12 which enabled us (18) P. de Mayo and H . Shizuka, J. Amer. Chem. SOC.,95, 3942 (1973). to study both reaction paths in detail without the need (19) Holder of a “Fundacion Juan March” Fellowship (Spain) for simplifying assumptions. 1972-1973. The conrotatory transition state was first located P. de Mayo,* R. Suaulg approximately by using the length (Y) of the breaking Pliotocliemistry Unit,Department of Chemistry bond as reaction ~ o o r d i n a t e ,and ~ the exact structure Unirersity of Western Ontario was then determined by a procedure,13 analogous in London, Ontario, Curiadu principle to that of McIver and Komornicki14 based on Rewired Jurze 6 , I974 a minimization of the scalar gradient of the energy. The final structure (Figure la) agrees closely with those predicted by M l N D 0 / 2 9 and by the ab initio calculaM I N D 0 / 3 Study of the Thermal tion of Buenker, et ~ 1 . ~ The 0 calculated activation Conversion of Cyclobutene to 1,3-Butadienel energy (49.0 kcal/mol) is too large (obsd 36 kcal/mol’j), Sir: a surprising discrepancy since MIND0/3 calculations While various qualitative arguments2-G lead to the for a number of other pericyclic processes have given activation energies that are within 1 5 kcal/mol from prediction that the thermal ring opening of cyclobutene experiment. The ab initio SCF calculations of Buen(1) to 1,3-butadiene (2) should take place preferentially in a conrotatory manner, a more quantitative ker, et af.,l0 gave a predicted activation energy of 65 kcal/mol without C1,l6 reduced to 48.8 kcal/mol1° by treatment is needed to establish the nature of the transiinclusion of extensive CI. tion state and the difference in activation energy between the “allowed” conrotatory and “forbidden” disAll previous studies (e.g., ref 7 and 10) of the disrotatory processes. rotatory opening of 1 to 2 have assumed that symmetry In our preliminary s t ~ d i e s ,using ~ MlND0/2,* we is retained throughout, i.e., that the two methylene established the unusual form of the disrotatory potengroups rotate at the same rate. Some restraint must tial surface, this having the “two-valley’’ structure charof course be imposed to force the reaction to follow a a c t e r i ~ t i cof~ ~“forbidden” pericyclic reactions, and also disrotatory path but this is unnecessarily severe. It was that the conrotatory transition state is n o n ~ l a n a r . ~ made necessary in the earlier calculations7~ lo by the need to reduce the number of independent variables. ( I ) This work was supported by the Air Force Office of Scientific Since the calculation of geometries for systems of this Research (Contract No. F44620-71-C-0119) and by the Robert A. Welch
Foundation (Grant N o . F-126). The calculations were carried out using the C D C 6400/6600 computer at the University of Texas Computation Center. (2) H. C. Longuet-Higgins and E. W. Abrahamson, J. Amer. Chem. SOC.,87,2045 (1965). (3) R . B. Woodward and R. Hoffmann, Angew. Chem., Int. Ed. Engl., 8,781 (1969). (4) M. J. S . Dewar, Angew. Chem., l n t . Ed. Engl., 10,761 (1971). 88, 1563, 1566 (1966). (5) H. Ztmmermann, J . Amer. Chem. SOC., (6) M. J. S. Dewar, S . Kirschner, and H. W. Kollmar, J. Amer. Chem. SOC..96. 5240 ( 1 974). (7j (a) M. J. S. Dewar and S. Kirschner, J . Amer. Chrm. Soc., 93, 4290 (1971); (b) ibid., 93,4291 (1971); (c) ibid., 93, 4292 (1971). (8) M. J. S. Dewar and D. Haselbach, J . Amer. Chem. Soc., 92, 590 (1970); N. Bodor, M. J. S. Dewar, A. Harget, and E. Haselbach, ibid., 3854 (1970). (9)‘M. J. S. Dewar, “XXJIIrd International Congress of Pure and Applied Chemistry, Special Lectures presented at Boston, U.S. A,, 2630 July 1971,”Vol. 1, Butterworths, London, 1971, p 1.
(10) (a) K. Hsu, R. 1. Buenker, and S. D. Peyerimhoff, J. Amer. Chem. SOC.,93, 2117 (1971); (b) R. J. Buenker, S. D . Peyerimhoff, and I
